Since we only call update_ref_write from a single place and we only call it
with onerr==QUIET_ON_ERR we can just as well get rid of it and just call
write_ref_sha1 directly. This changes the return status for _commit from
1 to -1 on failures when writing to the ref. Eventually we will want
_commit to start returning more detailed error conditions than the current
simple success/failure. For example if the commit failed due to name
conflicts etc.